1. Field of the Invention
This invention relates to a semiconductor memory device. More particularly, it relates to a semiconductor memory device adapted to testing and evaluation on the basis of a measurement of a write pulse width.
2. Description of the Related Art
The operating speed of semiconductor memory devices has been improved in recent years, and some of them operate at a pulse width of about 1 to 2 ns. On the other hand, testing equipment capable of generating such a high speed pulse is extremely expensive. Therefore, the evaluation of the circuits and their shipment and guarantee is expensive and the measurement of the write pulse width becomes difficult.
To avoid such problems, a write pulse generating circuit has been built into a semiconductor memory device (chip).
An example of such known methods fixes, hardware-wise, the write pulse width generated by the write pulse generating circuit at the time of production of the semiconductor memory device.
According to this system, however, the write pulse width cannot be changed and set after the production of the semiconductor memory device. For this reason, a simple judgement as to whether or not the product operates normally with the set write pulse width can be made, but the minimum pulse width that can be written cannot be measured. In other words, if data write cannot be made with the set write pulse width (that is, if the product does not operate normally), it is not at all possible to estimate how data write can be made possible by changing the pulse width or to what extent.
Another prior art method provides a large number of test terminals, and appropriately applies, from outside, a control signal to these terminals, and controls the built-in write pulse generating circuit so as to change the write pulse width.
In this system, however, there is a limit to the number of terminals on the semiconductor chip as a whole (that is, the number of external pins); hence, the number of terminals that can be used exclusively for testing is limited and it is difficult to accurately set the write pulse width. In other words, the adjustment can be made only roughly.